In the medium of paints, acrylics. Specifically Golden Open Acrylics (Slow-drying with a sort of oils appearance to them). Good for fine detail and blending, due to the protracted drying time, which is key since I use them mainly for portraiture.
In the medium of word, English. It's the only language I know, which helps in my preferring it. When I use it artistically, I use it in poetry and essays. I don't think of it as the most beautiful sounding language for poetry -- that would go to French, Japanese, or Chinese, for me. But I like its considerable flexibility, it's almost unsurpassed vocabulary, the ease with which new words can be incorporated into it, and so forth.
For essays, I like most of the same features as for poetry, but even more the precision and nuance of its vocabulary. Of course, like any language, there are some terms that we could really do with splitting their multiple meanings into a group of much more precise words, such as 'love'.